1. Field of the Invention
The present invention relates generally to precision optical inspection methods and apparatus, and more particularly to a method and apparatus for performing microscopic inspection and measurement of integrated circuit wafer geometry using interference microscopy in combination with electronic image processing.
2. Discussion of the Prior Art
It has long been desired that means be provided to inspect and measure the characteristics of microminiature surfaces such as those formed in integrated circuit wafers. One such characteristic of interest is the line widths of the various traces produced on a wafer surface during IC manufacture.
One prior art technique for integrated circuit metrology includes the use of an ordinary microscope with some form of electronic detector positioned at the image plane. For example, video cameras, scanning slits (see U.S. Pat. No. 4,373,817), shearing systems and linear arrays, have all been used as detectors with ordinary microscopes. However, the capability of the ordinary microscope is limited in that it can only measure the intensity of the optical wave amplitude and cannot measure the full complex amplitude including a magnitude as well as a phase. As a consequence, the three-dimensional nature of integrated circuit surfaces makes use of the classical microscope impractical for precision surface inspections and measurements of this type.
Other prior art techniques have used confocal laser scanning microscopes to obtain three dimensional data relating to integrated circuit surfaces. A rather thorough treatment of the subject may be found in T. Wilson and C. Shepard (1984), Theory and Practice of Scanning Optical Microscopy, Academic Press.
An improved prior art technique over the classical microscope is that used on what has become known as the Coherence Probe Microscope (See U.S. Pat. No. 4,818,110 Davidson and Davidson et al "An Application Of Interference Microscopy To Integrated Circuit Inspection And Metrology" Proc SPIE, vol. 775, 1987, both of which are expressly incorporated hereinto by reference.) The Coherence Probe Microscope is a form of optical microscope which uses optical images obtained from an interference microscope together with electronic processing in order to produce a synthetic image whose pixel grey scales are proportional to the absolute value of the mutual coherence function or the degree of coherence between light reflecting off of corresponding pixels in an interference microscope.
Whereas the Coherence Probe Microscope technique discussed above calculates the absolute value of the mutual coherence between the reference and object waves of a two beam interference microscope by acting directly on the images, the refinement presented here performs an additive transformation on the raw image data before calculating the coherence. The transformation adds in the signal from a single (or a set of) fixed pixels to the signals of all the other pixels. This transformation is done before calculating the absolute value of the mutual coherence, and effects a shift of the origin of the complex mutual coherence plane prior to taking the absolute value. This technique reduces some distortions in the image caused by transparent films or other causes. The distortion is reduced because the origin of the complex plane may be shifted away from the complex value of the mutual coherence for a point of interest, thereby reducing distortion caused by taking an absolute value, and desensitizing the measurement results to certain sytematic noise present in the mutual coherence data when measuring patterns in transparent films such as photoresist.